I think the aluminum head vette motors made around 285 hp from the factory, so with some chip tuning and a mild to medium cam 350 hp should be pretty easy. The big thing you get with the TPI is low end torque, which is great for cruising around and stop light fun. My understanding is they run out of airflow around 5200 to 5500rpm in stock form.
Truthfully, I am not certian 400 ponies are possible with the stock TPI. With the aftermarket base and runners, 400 hp, sure. That being said it is much easier to do the swap with a complete car to pull from instead of piecing it together like I did.
As for the tank you can run an extrenal fuel pump but I would recommend an in-tank pump with baffels. Some people use the B Body tanks from late Caprices, Road Masters etc, but that requires some trimming on the frame and cutting the trunck floor.You can get an after market tank with the baffles and pump but they are not cheap. There is also a shop that makes an retro-fit option, look up tanks.inc for that one. Me I took the tanks inc concept and madified my tank awith some parts I traded.
As for the wiring harness.. That's up to you if you want to spend the money for a prefabbed one or modify the orginal. I modified an orginal because I'm on a pretty tight budget but seriously looked into a prefabbed one off e-bay for the ease of it and fact that it would be new.
Send me a pm if you want I have dug up some decent web sties to get info from on this swap and a forum for modifing the stock TPI.
Steve
Here is how I modified my tank;
The pump setup is actually out of a Turbo couple T-Bird.
And here is a shot of mine.